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Author Topic: Wikipedia BRAS article?  (Read 3967 times)

Weaver

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Wikipedia BRAS article?
« on: September 04, 2016, 08:24:49 AM »

Referring to the Wikipedia page
        https://en.wikipedia.org/wiki/Broadband_remote_access_server

Is the penultimate paragraph, about protocols all a load of old codswallop? Or am I just losing it. (A: Quite probably the latter.  ;D )

Could anyone comment?
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kitz

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Re: Wikipedia BRAS article?
« Reply #1 on: September 04, 2016, 07:37:27 PM »

Seems about right to me - The penultimate paragraph is just talking about the encapsulation protocols that could be used between the modem/router and the bRAS... and the use of VCs/VLANs
 
Putting it into BTw/Openreach speak, for example:
  • 20CN the backhaul was ATM so we used PPPoA.  BTw used VP (Virtual Paths) to segment backhaul bandwidth between the DSLAMs and bRAS.
    There were only 10 bRAS dotted around the UK and the backhaul network from exchange to bRAS was called MiSP.  The whole MiSP backhaul network was ATM and hence the 155/622 Mb limitations.

  • NGA (FTTx) has GEA (Generic Ethernet Access) which uses PTM and PPPoE.  The cabs are connected to a Head-end exchange (not necessarily the local exchange) as Openreach has its own fibre spine that may not follow traditional E-side routing.   These head-end exchanges will most likely have its own MSE bRAS.  BTw uses S-VLANs (Service VLANs), C-VLANS (Customer VLANs) and also TVC-VLANs (TV connect/Multicast) to segment bandwidth between the exchange and GEA interconnect handover.

  • 21CN WBC for ADSL2+ is slightly more diverse.  Modem connects to a C.MSAN using ATM and PPPoA... but will at some point later in the chain will connect to an F.MSAN.  When and where depends upon location. They now appear to be rolling out MSE bRAS for all WBC (not just NGA) more locally.
     
  • BTw's Core uses MPLS.

Too much to explain in a couple of paras...  but more detail;-

20CN ATM Backhaul  (Bit dated now as that shows the old VPs based on 155Mbps pipes - it was originally written before wikipedia had info on such things, but it also goes into ATM, PPPoA, VPs and authentication in more depth).
BT's 21CN network - Shows the C.MSANs and F.MSANs for WBC/WBMC
BT's converged network - Shows how NGA, WBC and 20CN all slots together.  Note that image shows 21CN bRAS being at the old 20CN locations, prior to MSE bRAS. 
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Weaver

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Re: Wikipedia BRAS article?
« Reply #2 on: September 04, 2016, 08:45:22 PM »

@kitz - many thanks.

I really do need to read up on this properly somehow. An SIN somewhere?

A few questions, as I'm totally confused. If Ethernet frames are being passed in CPE-to-MSAN links, are they then passed on to a BRAS (a) intact, or (b) regenerated, or (c) stripped out? (PPPoA is still supported, so BRAS servers can't rely on receiving Ethernet frames, or can they? In the PPPoA case, does an MSAN just fake some Ethernet framing up?)

What Ethernet MAC addresses are used in the frames a BRAS receives?
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Weaver

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Re: Wikipedia BRAS article?
« Reply #3 on: September 04, 2016, 08:47:11 PM »

Article states
    “the BRAS is responsible for assigning network parameters such as IP addresses to the clients.”

Is that true? Certainly isn't true for me!
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ejs

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Re: Wikipedia BRAS article?
« Reply #4 on: September 04, 2016, 09:08:07 PM »

On 21CN, the only ATM is over the ADSL link between CPE and MSAN. If you're using PPPoA, the ATM gets removed at the MSAN and beyond that it's Ethernet, so presumably the MSAN puts the PPP data into Ethernet frames. If you're using PPPoE over the ADSL link, because the ADSL still actually carries ATM cells, you're really doing PPPoEoA. So the MSAN still has to reassemble the data out of the ATM cell payloads anyway.
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Weaver

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Re: Wikipedia BRAS article?
« Reply #5 on: September 04, 2016, 10:23:20 PM »

@ejs - understood. that's what I expected.

the IP address assignment thing, where did they get that from?
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Weaver

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Re: Wikipedia BRAS article?
« Reply #6 on: September 05, 2016, 06:07:50 AM »

Actually I stand corrected it seems that IP address assignment and even DHCP can be a function of a BRAS, even if it's sometimes turned off. Source: TR-092 spec
    https://www.broadband-forum.org/technical/download/TR-092.pdf

I was disagreeing with all of that Wikipedia article unreasonably, because the article seems to be talking always about what is a possible option, presumably according to config, whereas I was thinking about what actually happens in my particular case only.

And hallelujah TR-092 has some clear protocol stack diagrams in it including DSLAMs as well as BRAS. This dismisses a lot of the inaccurate statements about protocols which I have been hearing here and there.

For the first time, I have a picture of the complete end to end path of the higher level protocols below IP, but lacking detail on the various possible medium-dependent and link-dependent lower levels. Joy.
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kitz

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Re: Wikipedia BRAS article?
« Reply #7 on: September 05, 2016, 01:16:32 PM »

@ Weaver,  I started to type a lot of stuff last night but it got late, I got tired and went to bed. I see this morning you found another doc which I haven't yet read...  but since you were also asking about lower levels, I may have covered that in what I'd started to type, but I was going to split it into 2 sections and add in the image links.

I'll post what I started.. warts n' all but it may help slot everything together.

---------------

Quote
A few questions, as I'm totally confused. If Ethernet frames are being passed in CPE-to-MSAN links,

Thats why I said 21CN is more diverse.   There's C.MSANs (copper) and F.MSANs (fibre).  They may or may not be in the same location.
As I said above, the Modem connects to a C.MSAN using ATM and PPPoA .... & as ejs clarified, this is where ATM ends.  Because some exchanges may have both a C.MSAN and an F.MSAN and tier 1 exchanges also do WDM... it becomes more difficult to explain and why I linked to pages with the images...  but lets take the case where a small exchange only has a C.MSAN.

Modem to C.MSAN is ATM and PPPoA...  the backhaul traffic from this exchange will go to via a Layer 2 switch a larger exchange where there is an F.MSAN. The 21CN backhauls use ethernet (unlike the 20CN MiSP which still used ATM to the RAS).  So from the C.MSAN to F.MSAN its ethernet over SDH as the transport protocol.  Once at the F.MSAN, it then goes up another level (which may be at the same exchange.. or may be another larger exchange up the chain) where  WDM is used to make more efficient use of the fibre and increase capacity.


Quote
~ Exchange Backhaul from the MSAN

The backhaul transmission is SDH with Ethernet traffic encapsulated within GFP (Generic Framing Protocol) and protected using either SNCP or higher layer protection.

The network is tiered, which means that traffic will pass on up through various levels and nodes. MSANs are usually dual parented to the next level to provide some resiliency.

C.MSAN backhaul (Tier 3): Dual parenting to Fibre Access or WDM Access nodes unless impractical
F.MSAN backhaul (Tier 2): Dual homed. Either to WDM Access node and/or via other FMSAN(s) to WDM.
Depending upon the exchange and its size, the next tier may either be at the same location or at another exchange/PoP higher up in the chain.
Most exchanges (approx 4,400) will be Tier 2 & 3 which will be dual homed to the nearest WDM and/or via other F.MSANs to a WDM.


FTTx bypasses Tier 3 exchanges... and goes straight to Tier 2 Head-end exchanges using a separate Fibre Spine owned by Openreach which is already PPPoE.
In other words FTTC bypasses the ATM part and joins the 'ethernet network' in the same way that business links can do (depicted as the office building rather than a house).
C.MSANs use ATM
F.MSANs use Ethernet

« Last Edit: September 05, 2016, 01:33:51 PM by kitz »
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kitz

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Re: Wikipedia BRAS article?
« Reply #8 on: September 05, 2016, 01:30:39 PM »

Quote
the IP address assignment thing, where did they get that from?

They are talking about generic ISPs and thats how it would usually occur. 
But what you must bear in mind that BTw serves not just BT, but also many other ISPs... and therefore can and does use L2TP to hide things that sole providers wouldnt bother with.

Authentication occurs at the RAS using BT RADIUS servers.  So in geographic terms IPs are indeed allocated at the RAS.  However, BTw RADIUS 'talks' to the ISP RADIUS server to get an IP.
Sometimes if stage 2 ISP authentication fails, then you can find yourself issued with a BT IP address, but its useless in that you cant go anywhere outside of BTw's network.

There's some diagrams on the WBC/MBMC pages that show the BTw RADIUS and ISP RADIUS links during the authentication process.



Quote

Is that true? Certainly isn't true for me!

It's true.

The 20CN page also has a bit more info on Authentication and also a link that shows the full process where the RADiUS allocates the IP.   
You yourself phyically dont see this because AAISP uses shared WBMC... and BTw uses L2TP to hide much of whats going on in the background.   For shared WBMC you dont see anything before the ISP gateway.

If the ISP (such as BTr or Plusnet) uses dedicated WBMC, then the EU can usually still see the RAS IP as theres no L2TP to hide the RAS hops.


BTW, the new MSE bRAS IP allocation is quite interesting for WBMC dedicated ISPs... as the ISP allocates a pool of their IPs that each MSE bRAS can use, rather than one great big dynamic pool.   Remember MSE bRAS can be very close location-wise now for a lot of people.  What I dont particularly like about this is if say that pool only has a few hundred IPs then it can very closely identify your location if the person who had the IP before you was careless about geolocation tags.   Its also why some Plusnet users also moan that they can't pick up a different IP other than say the last couple of digits and may even be constantly given the same IP.
Plusnet Static IPs are slightly different and not dished out from the local pool.  It looks like they are using their steering server (again seen in the WBMC diagram) to direct to certain gateways and still use L2TP.
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Weaver

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Re: Wikipedia BRAS article?
« Reply #9 on: September 05, 2016, 07:32:48 PM »

Thank you so much for your very informative post, a lot of work put into a comprehensive and educational reply.

I suspect that we are both right, you and I, on this point, because I was talking both about a special case within a special case only, and also I was using a different sense of the word ‘allocated’. My LAN hosts’ IPv4 and IPv6 addresses are solely allocated on the basis of static configuration set in my router. No PPP involvement (and can't be, I'm not sure that's even doable using an FB 2000 series router). So in the case where radius where to fail, or change its mind, I would never even know. So do forgive my ignorance, I have never spotted the case you mentioned. I suppose I could have noted the IP address you get when you log in as bt_test_user (iirc) or the modern equivalent.

Of course, this is nothing to do with the article which uses misleading wording. The way you, _entirely reasonably_ read it, was _in the article's context, which was to mean 'BT_RADIUS' allocated, something of which I was totally ignorant. I have never looked into what a radius server does beyond vaguely knowing they are used for authenticating various types of logins.

So again many thanks, I have learned quite a bit. My opinion of the article has gone up a lot as I realise now that it's technically accurate. However it remains completely useless, as it's too vague, terms not defined, too scanty to be useful to someone like me and is potentially misleading. With a lot more detail, it would start to become useable. Wikipedia is really useless on the whole subject of Internet access and wholesaler / reseller access networks in general? To be fair to them there is the huge variability to worry about. But that isn't a sufficient excuse, doing one or two concrete networks and then adding caveats would at least give people some kind of picture of the possibilities.

No, Wikipedia = fail in general, sorry. Will have to go to kitz.co.uk instead  ;D
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